What the Hertzsprung-Russell Diagram Reveals about Stars

What the Hertzsprung-Russell Diagram Reveals about Stars The stars are the most amazing physical engines in the universe. They radiate light and heat, and they create chemical elements in their cores. However, when observers look at them in the night sky, all they see are thousands of pinpoints of light. Some appear reddish, others yellow or white, or even blue. Those colors actually give clues to the temperatures and ages of the stars and where they are in their life-spans. Astronomers sort stars by their colors and temperatures, and the result is a famous graph called the Hertzsprung-Russell Diagram. The H-R diagram is a chart that every astronomy student learns early on. Learning the Basic H-R Diagram Generally, the H-R diagram is a plot of  temperature vs. luminosity.  Think of luminosity as a way to define the brightness of an object. Temperature is something were all familiar with, generally as the heat   of an object. It helps define something called a stars spectral class, which astronomers also figure out by studying the wavelengths of light that come from the star. So, in a standard H-R diagram, spectral classes are labeled from hottest to coolest stars, with the letters O, B, A, F, G, K, M (and out to L, N, and R). Those classes also represent specific colors. In some H-R diagrams, the letters are arranged across the top line of the chart. Hot blue-white stars lie to the left and the cooler ones tend to be more toward the right side of the chart. The basic H-R diagram is labeled like the one shown here. The nearly diagonal line is called the main sequence. Nearly 90 percent of the stars in the universe exist along that line at one time in their lives. They do this while they are still fusing hydrogen to helium in their cores. Eventually, they run out of hydrogen and start to fuse helium.  Thats when they evolve to become giants and supergiants. On the chart, such advanced stars end up in the upper right corner. Stars like the Sun may take this path, and then ultimately shrink down to become white dwarfs, which appear in the lower left part of the chart. The Scientists and Science Behind the H-R Diagram The H-R diagram was developed in 1910 by the astronomers Ejnar Hertzsprung and Henry Norris Russell. Both men were working with spectra of stars- that is, they were studying the light from stars by using spectrographs. Those instruments break down the light into its component wavelengths. The way the stellar wavelengths appear gives clues to the chemical elements in the star. They can also reveal information about its temperature, motion through space, and its magnetic field strength. By plotting the stars on the H-R diagram according to their temperatures, spectral classes, and luminosity, astronomers can classify stars into their different types. Today, there are different versions of the chart, depending on what specific characteristics astronomers want to chart. Each chart has a similar layout, with the brightest stars stretching up toward the top and veering off to the top left, and a few in the lower corners. The Language of the H-R Diagram The H-R diagram uses terms that are familiar to all astronomers, so its worth learning the language of the chart. Most observers have probably heard the term magnitude when applied to stars. Its a measure of a stars brightness. However, a star might appear bright for a couple of reasons:   it could be fairly close and thus look brighter than one farther away; and  it could be brighter because its hotter. For the H-R diagram, astronomers are mainly interested in a stars intrinsic brightness- that is, its brightness due to how hot it actually is. Thats why luminosity (mentioned earlier) is plotted along the y-axis. The more massive the star is, the more luminous it is. Thats why the hottest, brightest stars are plotted among the giants and supergiants in the H-R Diagram. Temperature and/or spectral class are, as mentioned above, derived by looking at the stars light very carefully. Hidden within its wavelengths are clues about the elements are in the star. Hydrogen is the most common element, as shown by the work of astronomer Cecelia Payne-Gaposchkin in the early 1900s. Hydrogen is fused to make helium in the core, so thats why astronomers see helium in a stars spectrum, too. The spectral class is very closely related to a stars temperature, which is why the brightest stars are in classes O and B. The coolest stars are in classes K and M. The very coolest objects are also dim and small, and even include brown dwarfs. One thing to keep in mind is that the H-R diagram can show us what stellar type a star can become, but it doesnt necessarily predict any changes in a star. Thats why we have astrophysics - which applies the laws of physics to the lives of the stars.